This invention relates in general to active vehicular braking and suspension systems. In particular, this invention is concerned with detection of rough road conditions using suspension information and then adjusting active braking control for improved performance for the current road surface conditions.
Electronically-controlled active vehicular braking systems can include anti-lock braking (ABS), traction control (TC), and yaw stability control (YSC) functions. In such braking systems, sensors deliver input signals to an electronic control unit (ECU). The ECU sends output signals to electrically activated devices to apply, hold, and dump (relieve) pressure at wheel brakes of a vehicle. Electrically activated valves and pumps are used to control fluid pressure at the wheel brakes. Such valves and pumps can be mounted in a hydraulic control unit (HCU). The valves typically include two-state (on/off or off/on) solenoid valves and proportional valves.
A basic function of active braking systems is to detect wheel slip (e.g., skidding or loss of traction) and actuate the brakes (or reduce torque for the engine) in a manner to reduce or control wheel slip. An individual wheel speed is measured and wheel slip is detected by comparing the individual wheel speed to a target speed determined for that wheel. Various control parameters of the active braking systems are chosen to provide satisfactory performance over all conditions that are encountered during operation. For example, activation of the active control (e.g., ABS or TC) to control slip does not occur until the difference between actual wheel speed and target speed exceeds a slip threshold. A base threshold is chosen that achieves best overall performance for all conditions.
Certain assumptions or tradeoffs are made in selecting a base threshold. For example, the flatness or roughness of the road surface influences the amount of slip that will achieve the highest overall vehicle acceleration or deceleration. Thus, to achieve a shortest stopping distance, there is an optimum slip threshold. Since characterization of road surface condition is not available to prior art systems, the base threshold is chosen for achieving best overall stopping distances.
It is known to dynamically vary this slip threshold in response to certain characteristics of the wheel speed signals (e.g., acceleration changes) to either increase or decrease the amount of slip that is controlled. For example, wheel speed signals have been analyzed in attempts to detect wheel hop, but this has not led to accurate road surface classification.
Electronically-controlled suspension systems typically include semi-active suspension systems and active suspension systems to provide active damping for a vehicle. In such suspension systems, sensors deliver input signals to an electronic control unit (ECU). The ECU sends output signals to electrically activated devices to control the damping rate of the vehicle. Such devices include actuators to control fluid flow and pressure. The actuators typically include electrically activated valves such as two-state digital valves and proportional valves.